Electric double layer capacitor

ABSTRACT

An electric double layer capacitor having a pair of polarized electrodes and an electrolytic solution capable of forming an electric double layer at the interface with the polarized electrodes, wherein the electrolytic solution is an organic electrolytic solution containing a fluorobenzene of the formula 1:  
                 
 
     wherein n is an integer of from 1 to 6.

[0001] The present invention relates to an electric double layercapacitor, particularly to an electric double layer capacitor havinghigh voltage retention and excellent reliability.

[0002] As a shape of a conventional electric double layer capacitor,there may be a coin type wherein an element having a separatorsandwiched between a pair of polarized electrodes composed mainly ofactivated carbon formed on current collectors, is accommodated togetherwith an electrolytic solution in a metal casing, which is then sealed bya metal cover via a gasket, or a cylindrical type wherein an elementhaving a pair of polarized sheet electrodes wound with a separatorinterposed therebetween, is accommodated together with an electrolyticsolution in a metal casing, which is then sealed so that theelectrolytic solution will not evaporate from an opening of the casing.

[0003] Further, as one for a large current and large capacitance, alamination type electric double layer capacitor has also been proposedwherein an element having many polarized sheet electrodes laminated viaa separator disposed therebetween, is incorporated (JP-A-4-154106,JP-A-3-203311, JP-A-4-286108). Namely, rectangular polarized sheetelectrodes are used as a positive electrode and a negative electrode,and they are alternately laminated with a separator interposedtherebetween, to form an element, which is then accommodated in a casingin such a state that a positive electrode lead member and a negativeelectrode lead member are connected by caulking to the terminals of thepositive and negative electrodes, respectively, then the element isimpregnated with an electrolytic solution, and the casing is closed witha cover.

[0004] As an electrolytic solution for a conventional electric doublelayer capacitor, not only an aqueous electrolytic solution containing amineral acid such as sulfuric acid, an alkali metal salt or an alkali,but also various organic electrolytic solutions have been used. As thesolvent for such organic electrolytic solutions, propylene carbonate,γ-butyrolactone, acetonitrile, dimethyl formamide (JP-A-49-068254) or asulfolane derivative (JP-A-62-237715), has been known. When thewithstand voltages are compared, the aqueous electrolytic solution has awithstand voltage of 0.8 V, while the organic electrolytic solution hasa withstand voltage of from 2.5 to 3.3 V. The electrostatic energy of acapacitor corresponds to the square of the withstand voltage.Accordingly, from the viewpoint of the electrostatic energy, the organicelectrolytic solution is more advantageous.

[0005] The withstand voltage of an electric double layer capacitor isbasically restricted by the electrochemical decomposition voltage of theelectrolytic solution. In a case where an organic electrolytic solutionwhich has a high withstand voltage as compared with an aqueouselectrolytic solution, is used as an electrolytic solution, it is usedby an application of a voltage higher than the decomposition voltage ofwater, whereby electrolysis takes place due to an impurity, particularlywater, contained in the electrolytic solution. Accordingly, it is commonto use an organic electrolytic solution as dehydrated and having boththe solvent and electrolyte highly purified.

[0006] On the other hand, for the electrodes for an electric doublelayer capacitor, an electrode material having a high specific surfacearea, is used, but when it is combined with the above organicelectrolytic solution, it is common to use activated carbon as theelectrode material. Activated carbon is a porous material having finepores of a few nm, but has high adsorbing ability and thus is likely toadsorb moisture in an environment. Accordingly, activated carbon isrequired to be highly dehydrated in the process for producing anelectric double layer capacitor. It is usually required to carry outdehydration treatment at a high temperature of at least 300° C. invacuum or in an inert gas atmosphere in order to completely removemoisture from fine pores of activated carbon. However, activated carbonparticles are usually formed on a current collector by means of a bindersuch as an organic polymer to constitute an electrode, and the binderundergoes thermal decomposition by treatment at a high temperature of atleast 300° C. Accordingly, heat treatment is usually carried out at atemperature of at most 200° C., whereby it is difficult to completelyremove moisture in the activated carbon electrodes.

[0007] Further, there has been a problem that due to the presence of aportion where the activated carbon surface and the electrolytic solutionare not in contact with each other, i.e. the activated carbon surface isnot wetted by the electrolytic solution due to inadequate impregnationof the electrolytic solution, or a portion where a gas generated byelectrolysis is retained within fine pores of activated carbon, the areato store electric charge tends to be small, the capacitance developingratio tends to decrease, and the resistance tends to increase.

[0008] An electric double layer capacitor employing an organicelectrolytic solution, is operated at a voltage of at least 2 V which ishigher than the theoretical decomposition voltage (1.23 V) of water forthe purpose of increasing the energy density. Accordingly, in a statewhere the voltage is applied after assembling a capacitor cell, waterremaining in the above-mentioned fine pores will be electrolyzed togenerate a gas. It has been found that the generated gas will begradually accumulated in the fine pores of activated carbon and willremain in the interior of the element without being discharged out ofthe element formed by impregnating the electrolytic solution to a pairof electrodes facing each other via a separator. If the electric doublelayer capacitor is used for a long period of time in such a state, theelectrolytic solution present in the pores of activated carbon is likelyto be driven out by the generated gas, whereby the capacitance normallyexpected to be obtainable, tends to be hardly obtainable, and further,an electroconductive path created by movement of ions in the pores, islikely to be blocked off. Consequently, there will be a decrease in thecapacitance of the electric double layer capacitor or a deterioration inperformance such as an increase of the internal resistance. Further,water remaining in the pores can not completely be removed, electrolysisdue to adsorbed electric charge will take place continuously.Accordingly, there has been a problem that the voltage retention is poorafter applying a voltage and opening the circuit.

[0009] Under the circumstances, it is an object of the present inventionto solve the above-mentioned problems of the prior art and to provide anelectric double layer capacitor having high voltage retention andexcellent reliability, and an organic electrolytic solution for such apurpose.

[0010] The present invention provides an electric double layer capacitorhaving a pair of polarized electrodes and an electrolytic solutioncapable of forming an electric double layer at the interface with thepolarized electrodes, wherein the electrolytic solution is an organicelectrolytic solution containing a fluorobenzene of the formula 1:

[0011] wherein n is an integer of from 1 to 6.

[0012] Further, the present invention provides an organic electrolyticsolution comprising a fluorobenzene of the formula 1:

[0013] wherein n is an integer of from 1 to 6, and an electrolyte havingat least one cation selected from the group consisting of the formula 2:

R¹R²R³R⁴N⁺  Formula 2

[0014] wherein each of R¹, R², R³ and R⁴ which are independent of oneanother, is a methyl group, an ethyl group or a n-propyl group, providedthat two selected from R¹ to R⁴ may together form a tetramethylenegroup, the formula 3:

[0015] wherein each of R⁵ and R⁶ which are independent of each other, isa C₁₃ alkyl group, and the formula 4:

R⁷R⁸R⁹R¹⁰N⁺  Formula 4

[0016] wherein R⁷ is a methoxyalkyl group of the formula —(CH₂)_(n)OCH₃,wherein n is an integer of from 1 to 3, and each of R⁸, R⁹ and R¹⁰ whichare independent of one another, is a methyl group or an ethyl group,provided that two selected from R⁸ to R¹⁰ may together form atetramethylene group.

[0017] Now, the present invention will be described in detail withreference to the preferred embodiments.

[0018] In the present invention, the fluorobenzene is preferably onehaving a large dielectric constant. Preferred is at least one memberselected from the group consisting of a monofluorobenzene, adifluorobenzene and a trifluorobenzene. As the structure of thedifluorobenzene, o-difluorobenzene or m-difluorobenzene is preferred.Likewise, as the structure of the trifluorobenzene,1,2,3-trifluorobenzene or 1,2,4-trifluorobenzene is preferred.

[0019] In the present invention, the amount of fluorobenzene in theelectrolytic solution is suitably adjusted depending upon the porecharacteristics or the amount of water remaining in the carbon materialcontained in the electrodes. However, the fluorobenzene is preferablymaintained in a state where it is completely dissolved in the organicelectrolytic solution. Further, when the fluorobenzene is added, thedielectric constant of the organic electrolytic solution decreases, andaccordingly, the amount of the fluorobenzene is preferably controlled tobe within a range where decrease of the ion conductivity due to adecrease in the dielectric constant of the organic electrolyticsolution, is small. Thus, the fluorobenzene is contained preferably inan amount of from 0.1 to 30%, particularly from 1 to 20%, in the totalmass of the electrolytic solution.

[0020] The operational principle in the present invention is not clearlyunderstood, but it is considered that the fluorobenzene contained in theelectrolytic solution has a high affinity to the pseudographite surfacepresent on the inner wall of pores of the carbon material and can easilybe substituted and adsorbed for the water remaining in the fine poreswithout being removed by the heat treatment. When a voltage is appliedto an element having an electrolytic solution impregnated, such waterwill readily be electrolyzed and gasified. Accumulation of electriccharge due to formation of an electric double layer, takes place mainlyin the pores. Accordingly, with a conventional electric double layercapacitor containing no fluorobenzene in the electrolytic solution, if avoltage is applied to the element having the electrolytic solutionimpregnated, a gas generated by electrolysis of water remaining in thefine pores, will remain in the fine pores, whereby the performance usedto be deteriorated. Whereas, in the present invention, it is consideredthat the gas generated by the electrolysis of remaining water by anapplication of a voltage will be present not in fine pores as describedabove, but outside the fine pores i.e. in spaces among particles or inmicropores in the activated carbon particles, or will be discharged asbubbles out of the element. Accordingly, it is considered thatdeterioration in performance of the electric double layer capacitor canbe suppressed to a minimum level.

[0021] Such an effect is observed also in a case where benzene or itschlorinated derivative is added to the electrolytic solution and isdisclosed in JP-A-2000-252171. Although the operation principle is notclearly understood, by the presence of a compound having a benzene ring,the affinity of the electrolytic solution to the activated carbonsurface is improved, and further due to a strong electron attractingproperty of a fluoro group, the solvent containing a fluorobenzene willby itself show a high dielectric constant. It is accordingly consideredthat the fluorobenzene shows a higher compatibility to an organicsolvent of the organic electrolytic solution and a higher effect thanbenzene or its chlorinated derivative.

[0022] As described above, when a voltage is applied to the aboveelement, a decomposed gas will be generated, and this gas will increasethe internal pressure of the electric double layer capacitor cell.Accordingly, the application of a voltage in the production process ispreferably carried out in an open state in a dried atmosphere, and thegenerated gas is discharged out of the capacitor cell. Here, “an openstate” means a state where the element is not accommodated in the cell,or a state where even if it is accommodated in the cell, the cell is notclosed. Here, the dried atmosphere is preferably one having a dewpointof at most −20° C., more preferably at most −30° C., still morepreferably at most −40° C.

[0023] Further, the voltage to be applied to the element, is preferablyat least 2 V which is higher than the decomposition voltage of water,more preferably at least 2.5 V. The temperature at which the voltage isapplied to the element, is preferably from 15 to 85° C., more preferablyfrom 20 to 70° C. If the voltage is applied under heating, the effect toincrease the durability of the electric double layer capacitor will belarge, and the voltage application time can be shortened. However, ifthe temperature is too high, the initial capacitance tends to decrease,and the internal resistance tends to increase.

[0024] The electrolyte to be used for the electrolytic solution for theelectric double layer capacitor of the present invention is particularlypreferably one having at least one cation selected from the groupconsisting of a quaternary onium cation of the formula 2:

R¹R²R³R⁴N⁺  Formula 2

[0025] wherein each of R¹, R², R³ and R⁴ which are independent of oneanother, is a methyl group, an ethyl group or a n-propyl group, providedthat two selected from R¹ to R⁴ may together form a tetramethylenegroup, an imidazolium cation of the formula 3:

[0026] wherein each of R⁵ and R⁶ which are independent of each other, isa C₁₋₃ alkyl group, and a quaternary onium cation of the formula 4:

R⁷R⁸R⁹R¹⁰N⁺  Formula 4

[0027] wherein R⁷ is a methoxyalkyl group of the formula —(CH₂)_(n)OCH₃,wherein n is an integer of from 1 to 3, and each of R⁸, R⁹ and R¹⁰ whichare independent of one another, is a methyl group or an ethyl group,provided that two selected from R⁸ to R¹⁰ may together form atetramethylene group.

[0028] Further, the anion is preferably an anion selected from the groupconsisting of BF₄ ⁻, PF6⁻, CF₃SO₃ ⁻ and (CF₃SO₂)₂N⁻. Particularly, BF₄ ⁻is more preferred from the viewpoint of the electrical conductivity andthe electrochemical stability.

[0029] The concentration of the above electrolyte in the electrolyticsolution is preferably at least 0.5 mol/kg, particularly preferably atleast 1.0 mol/kg, for the purpose of securing the amount of ionsrequired for formation of an electric double layer and obtainingadequate electroconductivity.

[0030] As the organic solvent to be used in the present invention, aknown solvent may be used. For example, a solution is preferred whichcontains, as a solvent, at least one organic solvent, such as, a cycliccarbonate such as propylene carbonate, ethylene carbonate or butylenecarbonate, a chain carbonate such as dimethyl carbonate, ethylmethylcarbonate or diethyl carbonate, a cyclic lactone such as γ-butyrolactoneor γ-valerolactone, a nitrile such as acetonitrile or glutaronitrile, asulfolane derivative such as sulfolane or 3-methylsulfolane,dimethylformamide, 1,2-dimethoxyethane, nitromethane, ortrimethylphosphate. It is particularly preferably at least one memberselected from the group consisting of propylene carbonate, butylenecarbonate, sulfolane, dimethyl carbonate and methylethyl carbonate.

[0031] The organic electrolytic solution comprising the aboveelectrolyte, the solvent and the fluorobenzene, preferably containsmetal impurities and water as little as possible. Usually, one having awater content of at most 10 ppm, is preferably employed.

[0032] In the present invention, it is preferred to employ an organicelectrolytic solution wherein an electrolyte of the formula 5 or 6:

[0033] is contained in an amount of from 30 to 60%, monofluorobenzene iscontained in an amount of from 0.1 to 30%, and dimethyl carbonate iscontained in an amount of from 20 to 69%, in the total mass of theelectrolytic solution, whereby a high electroconductivity is obtainableat a level of from 13 to 18 mS/cm with the electrolyte of the formula 5or from 15 to 20 mS/cm with the electrolyte of the formula 6, thereliability is excellent even when a high voltage at a level of 3.0 V isapplied, and the increase in resistance can be suppressed. Further, itis more preferred to employ an organic electrolytic solution whereinethylmethyl carbonate is contained in an amount of from 0.1 to 30% inthe total mass of the electrolyte, whereby the low temperaturecharacteristics can be improved.

[0034] Further, it is preferred to employ an organic electrolyticsolution wherein an electrolyte of the formula 7:

[0035] is contained in an amount of from 15 to 60%, monofluorobenzene iscontained in an amount of from 0.1 to 30%, and propylene carbonate iscontained in an amount of from 10 to 85%, in the total mass of theelectrolytic solution, whereby a high electroconductivity of from 15 to23 mS/cm can be obtained, and a high output discharge can be carriedout.

[0036] The polarized electrodes to be used for the electric double layercapacitor of the present invention may be ones made mainly of anelectrochemically inactive material having a high specific surface area,specifically those made mainly of activated carbon, fine metal particlesor fine electrically conductive oxide particles. Among them, it ispreferred to use those having an electrode layer comprising a powder ofcarbon material having a high specific surface area such as activatedcarbon, formed on the surface of metal current collectors.

[0037] Specifically, the electrode layer is formed preferably by using,as the main component, a powder of carbon material such as activatedcarbon or polyacene having a large specific surface area (specificsurface area: about 200 to 3,000 m²/g), adding thereto carbon black,acetylene black, Ketjenblack or carbon whisker as a conductive material,and polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) orcarboxymethylcellulose as a binder, kneading the mixture in the presenceof a liquid lubricant such as an alcohol, molding the mixture into asheet by rolling, followed by drying to obtain a sheet-form moldedproduct, which is bonded by heat pressing or bonded by means of anelectrically conductive adhesive or the like to both sides of a metalcurrent collector.

[0038] Further, instead of kneading, a solvent capable of dissolving theabove binder or a solvent mixture containing such a solvent (water,N-methylpyrrolidone, etc.) may be mixed with activated carbon, aconductive agent and a binder to obtain a slurry, which may be coated onboth sides of a metal current collector and dried to form the electrodelayer. The thickness of such an electrode layer is not particularlylimited, but is usually from about 10 um to 0.5 mm.

[0039] As the activated carbon material, one derived from a naturalplant tissue such as coconut shell, a synthetic resin such as a phenolicresin or a fossil fuel such s coal, coke or pitch, may be employed. Asan activating method for activated carbon, steam activation or alkaliactivation (particularly activation by KOH) may be applied, although itmay vary depending upon the raw material to be employed. Activatedcarbon derived from a natural plant tissue or a fossil fuel, contains arelatively large amount of metal impurities, and accordingly, washingwith e.g. an acid is usually required. Similarly, activated carbonobtained by alkali activation contains a large amount of an alkali metalused for the activation or metal impurities brought from an activationapparatus due to the reaction with the alkali, and accordingly a washingoperation will be required. Among them, steam activated carbon made of asynthetic resin as a raw material, is most preferred from the viewpointof metal impurities.

[0040] The element construction of the electric double layer capacitorof the present invention is not particularly limited, and the presentinvention can be applied to any one of a coin type structure, acylindrical structure or an angular structure. For example, the cointype structure may be formed in such a manner that an element is formedby disposing a separator between a pair of electrodes having electrodelayers composed mainly of activated carbon provided on currentcollectors, and the element is, together with an electrolytic solution,sealed in a coin type metal casing by a metal cover and a gasket whichinsulates both.

[0041] Whereas, the cylindrical structure may be formed in such a mannerthat a pair of strip-shaped electrodes, specifically a strip-shapedpositive electrode having an electrode layer composed mainly of e.g.activated carbon formed on both sides of a metal current collector and astrip-shaped negative electrode having an electrode layer of the sameconstruction formed on both sides of a metal current collector, arealternately laminated via a strip-shaped separator and wound to obtain awound element, which is then accommodated in a cylindrical metal casingand impregnated with the electrolytic solution, whereupon the currentcollecting leads taken out from the positive electrode and the negativeelectrode, respectively, are connected, respectively, to the electrodeterminals provided, for example, on an electrically insulating sealingcover, and the sealing cover is fit to the metal casing.

[0042] The angular structure may be formed in such a manner thatelectrode layers are formed on both sides of a rectangular metal currentcollector, a plurality of positive electrodes and a plurality ofnegative electrodes, each provided with a current collecting lead, arealternately laminated via a separator, to form a laminated elementhaving current collecting leads taken out, which is accommodated in anangular metal casing and impregnated with the electrolytic solution,whereupon a sealing cover is fit on the angular casing.

[0043] The current collector may be made of any metal so long as it iselectrochemically or chemically corrosion resistant. In the case of acoin type structure, the housing member such as the metal sealing coveror the metal casing, may serve as a current collector, in many cases. Asthe current collector in the case of the cylindrical structure or theangular structure, it is preferred to employ a surface-roughened foil ornet made of a metal such as aluminum, stainless steel, nickel ortantalum, particularly a foil or net made of a stainless steel, aluminumor an alloy containing it. More preferred is an aluminum foil having apurity of 99.9%, particularly preferably 99.99%. In the presentinvention, it is preferred to employ a metal current collector made ofsuch a metal foil and having a thickness of from 10 um to 0.5 mm.

[0044] In the case of a cylindrical structure or an angular structure,current collecting leads will be provided to the metal currentcollectors. It is preferred to provide a tape- or ribbon-shaped portionon a current collector having no electrode layer formed thereon and tobond an electrically conductive tab terminal, wire, tape, ribbon or thelike by e.g. welding to such a portion to form a current collectinglead. Otherwise, a portion having no electrode layer formed, is providedat a part of a current collector, so that such a portion may be used asa current collecting lead. Specifically, for example, in the case of acylindrical structure, a strip portion having no electrode layer formed,may be provided along one end in the longitudinal direction of a stripcurrent collector, and the counter electrode is overlaid via a separatorso that the strip portion is located at the opposite end, and theassembly is wound to obtain an element, whereby both end surfaces (theabove strip portions) of the element can be used as current collectingleads.

[0045] The separator of the present invention is not particularlylimited, so long as it is a porous separator so that ions can permeatetherethrough. A fine porous polyethylene film, a fine porouspolypropylene film, a polyethylene non-woven fabric, a polypropylenenon-woven fabric, a glass fiber incorporated non-woven fabric, a glassmat, cellulose paper, sisal hemp or Manila hemp, may, for example, bepreferably employed. The thickness of the separator is preferably from20 to 200 μm, particularly preferably from 30 to 100 μm. From theviewpoint of the absorptivity for the electrolytic solution, the liquidmaintaining property and the internal resistance, the higher theporosity, the better. However, as the porosity is high, defects such aspinholes are likely to increase, thus leading to self discharge failure.Accordingly, the porosity is usually preferably within a range of from50 to 90%, more preferably within a range of from 60 to 85%.

[0046] Now, the present invention will be described in further detailwith reference to Examples and Comparative Examples. However, it shouldbe understood that the present invention is by no means restricted bysuch specific Examples.

EXAMPLE 1 Present Invention

[0047] Ethanol was added to a mixture comprising a phenol resin typeactivated carbon having a specific surface area of 2,000 m²/g activatedby steam, PTFE and carbon black in a mass ratio of 8:1:1, followed bykneading. This was formed into a sheet shape and then rolled in athickness of 0.6 mm to obtain an electrode sheet, which was punched intodisks having a diameter of 12 mm.

[0048] Such disk-shaped electrodes were bonded to the positive electrodeside and negative electrode side insides, respectively, of a stainlesssteel casing serving as a current collector and housing member for acoin-shaped cell by means of a graphite type conductive adhesive. Then,the entire assembly including the stainless steel casing was subjectedto heat treatment under reduced pressure to remove moisture, etc. Theelectrodes were impregnated with an electrolytic solution having 1.5mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in a solvent mixture comprisingpropylene carbonate and monofluorobenzene in a mass ratio of 95:5. Aseparator (thickness: 160 μm, porosity: 70%) of a non-woven fabric madeof polypropylene fiber was sandwiched between the two electrodes, andthe stainless steel casing was caulked by a gasket as an insulator andsealed, to obtain a coin-shaped electric double layer capacitor having adiameter of 18.4 mm and a thickness of 2.0 mm.

EXAMPLE 2 Present Invention

[0049] A coin-shaped electric double layer capacitor was obtained in thesame manner as in Example 1 except that as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in a solventmixture comprising propylene carbonate and 1,2-difluorobenzene in a massratio of 96:4, was used.

EXAMPLE 3 Present Invention

[0050] Ethanol was added to a mixture comprising a phenol resin typeactivated carbon having a specific surface area of 2,000 m²/g, activatedby molten KOH, PTFE and carbon black in a mass ratio of 8:1:1, followedby kneading and forming into a sheet shape, and then by rolling in athickness of 0.1 mm to obtain a strip electrode sheet. The obtainedelectrode sheet was bonded by an electrically conductive adhesive to analuminum foil having the surface etched. Then, moisture, etc. wereremoved by heat treatment under reduced pressure, and a glass fiberseparator (thickness: 100 μm, porosity: 80%) was sandwiched between thepositive and negative electrodes and wound up on a winding core having adiameter of 2 mm to obtain a cylindrical element having a diameter of 7mm and a height of 20 mm. This element was impregnated with anelectrolytic solution having 1.2 mol/kg of (C₂H₅)₄N⁺BF₄ ⁻ dissolved in asolvent mixture comprising acetonitrile and 1,2,3-trifluorobenzene in amass ratio of 97:3, and butyl rubber was inserted and sealing wascarried out by a caulking tool to obtain a cylindrical electric doublelayer capacitor.

EXAMPLE 4 Present Invention

[0051] Ethanol was added to a mixture comprising activated carbon havinga specific surface area of 2,000 m²/g, obtained by calcining a resolresin in a nitrogen atmosphere at 650° C. and activated with molten KOH,PTFE and carbon black in a mass ratio of 8:1:1, followed by kneading andforming into a sheet shape, and then by rolling in a thickness of 0.6 mmto obtain an electrode sheet, which was punched into disks having adiameter of 12 mm.

[0052] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm was obtained in the same manner asin Example 1 except that such disk-shaped electrodes were used aspositive and negative electrodes, and as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in a solventmixture comprising sulfolane, ethylmethyl carbonate andmonofluorobenzene in a mass ratio of 85:15:5, was used.

EXAMPLE 5 Present Invention

[0053] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm was obtained in the same manner asin Example 1 except that as the electrolytic solution, a solution having2.2 mol/kg of the salt of the above formula 5 dissolved in a solventmixture comprising dimethyl carbonate and monofluorobenzene in a massratio of 60:40, was used.

EXAMPLE 6 Present Invention

[0054] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm, was obtained in the same manner asin Example 1, except that as the electrolytic solution, a solutionhaving 2.2 mol/kg of the salt of the above formula 5 dissolved in asolvent mixture comprising dimethyl carbonate, ethylmethyl carbonate andmonofluorobenzene in a mass ratio of 60:10:30, was used.

EXAMPLE 7 Present Invention

[0055] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm, was obtained in the same manner asin Example 1, except that as the electrolytic solution, a solutionhaving 2.4 mol/kg of the salt of the above formula 6 dissolved in asolvent mixture comprising dimethyl carbonate and monofluorobenzene in amass ratio of 60:40, was used.

EXAMPLE 8 Present Invention

[0056] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm, was obtained in the same manner asin Example 1, except that as the electrolytic solution, a solutionhaving 2.4 mol/kg of the salt of the above formula 6 dissolved in asolvent mixture comprising dimethyl carbonate, ethylmethyl carbonate andmonofluorobenzene in a mass ratio of 60:10:30, was used.

EXAMPLE 9 Present Invention

[0057] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm, was obtained in the same manner asin Example 1, except that as the electrolytic solution, a solutionhaving 2.5 mol/kg of the salt of the above formula 7 dissolved in asolvent mixture comprising propylene carbonate and monofluorobenzene ina mass ratio of 80:20, was used.

EXAMPLE 10 Comparative Example

[0058] A coin-shaped electric double layer capacitor was obtained in thesame manner as in Example 1 except that as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in apropylene carbonate solvent, was used.

EXAMPLE 11 Comparative Example

[0059] A coin-shaped electric double layer capacitor was obtained in thesame manner as in Example 1 except that as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in apropylene carbonate solvent, was used.

EXAMPLE 12 Comparative Example

[0060] A cylindrical electric double layer capacitor was obtained in thesame manner as in Example 3 except that as the electrolytic solution, asolution having 1.2 mol/kg of (C₂H₅)₄N⁺BF₄ ⁻ dissolved in anacetonitrile solvent, was used.

EXAMPLE 13 Comparative Example

[0061] A coin-shaped electric double layer capacitor was obtained in thesame manner as in Example 4 except that as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in a solventmixture comprising sulfolane and ethylmethyl carbonate in a mass ratioof 8:2, was used.

EXAMPLE 14 Comparative Example

[0062] A coin-shaped electric double layer capacitor was obtained in thesame manner as in Example 1 except that as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in a solventmixture comprising propylene carbonate and benzene in a mass ratio of95:5, was used.

EXAMPLE 15 Comparative Example

[0063] A coin-shaped electric double layer capacitor was obtained in thesame manner as in Example 1 except that as the electrolytic solution, asolution having 1.5 mol/kg of (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ dissolved in a solventmixture comprising propylene carbonate and monochlorobenzene in a massratio of 95:5, was used.

EXAMPLE 16 Comparative Example

[0064] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm was obtained in the same manner asin Example 1 except that as the electrolytic solution, a solution having2.2 mol/kg of the salt of the above formula 5 dissolved in a dimethylcarbonate solvent, was used.

EXAMPLE 17 Comparative Example

[0065] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm was obtained in the same manner asin Example 1 except that as the electrolytic solution, a solution having2.4 mol/kg of the salt of the above formula 6 dissolved in a dimethylcarbonate solvent, was used.

EXAMPLE 18 Comparative Example

[0066] A coin-shaped electric double layer capacitor having a diameterof 18.4 mm and a thickness of 2.0 mm was obtained in the same manner asin Example 1 except that as the electrolytic solution, a solution having2.5 mol/kg of the salt of the above formula 7 dissolved in a propylenecarbonate solvent, was used.

[0067] Evaluation

[0068] The compositions of the respective electrolytic solutions inExamples 1 to 9 and Examples 10 to 18, and the voltage retention afterthe electric double capacitors having the voltage as identified inTables 1 and 2 applied, were maintained in an open circuit for 72 hours,are shown in Tables 1 and 2, respectively. To make the comparison inperformance clear, the applied voltages were adjusted to be the same inExamples 1, 10, 14 and 15, in Examples 2 and 11, in Examples 3 and 12,in Examples 4 and 13, in Examples 5, 6 and 16, in Examples 7, 8 and 17,and in Examples 9 and 18, respectively. Further, the same voltages asfor the evaluation of the voltage retention, were applied to theelectric double layer capacitors of Examples 1 to 18, whereby theinitial capacitance and the internal resistance were measured, andfurther the change in capacitance after maintaining them in a constanttemperature and humidity chamber of 70° C. for 1,000 hours, wasmeasured. The results are shown in Table 3. TABLE 1 Composi- VoltageComponents of the tional ratio Voltage retention electrolytic solution(%) (V) (%) Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 2.5 94 1 Propylene carbonate66.1 Monofluorobenzene 3.5 Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 2.8 92 2Propylene carbonate 66.8 1,2-Difluorobenzene 2.8 Ex. (C₂H₅)₄N⁺BF₄ ⁻ 26.02.5 91 3 Acetonitrile 71.8 1,2,3-Trifluorobenzene 2.2 Ex.(C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 3.0 92 4 Sulfolane 55.7 Ethylmethyl carbonate10.4 Monofluorobenzene 3.5 .Ex. 5

47.3 3.0 91 Dimethyl carbonate 31.6 Monofluorobenzene 21.1 .Ex. 6

47.3 3.0 Dimethyl carbonate 31.6 Ethylmethyl carbonate 5.3Monofluorobenzene 15.8 .Ex. 7

52.1 3.0 90 Dimethyl carbonate 28.7 Monofluorobenzene 19.2 .Ex. 8

52.1 3.0 91 Dimethyl carbonate 28.7 Ethylmethyl carbonate 4.8Monofluorobenzene 14.4 .Ex. 9

49.5 2.5 93 Propylene carbonate 40.4 Monofluorobenzene 10.1

[0069] TABLE 2 Composi- Voltage Components of the tional ratio Voltageretention electrolytic solution (%) (V) (%) Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.42.5 89 10 Propylene carbonate 69.6 Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 2.8 8511 Propylene carbonate 69.6 Ex. (C₂H₅)₄N⁺BF₄ ⁻ 26.0 2.5 85 12Acetonitrile 74.0 Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 3.0 86 13 Sulfolane 55.7Ethylmethyl carbonate 13.9 Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 2.5 90 14Propylene carbonate 66.1 Benzene 3.5 Ex. (C₂H₅)₃(CH₃)N⁺BF₄ ⁻ 30.4 2.5 9215 Propylene carbonate 66.1 Monochlorobenzene 3.5 H₅)Ex. 16

47.3 3.0 87 Dimethyl carbonate 52.7 H₅)Ex. 17

52.1 3.0 87 Monofluorobenzene 47.9 H₅)Ex. 18

49.5 2.5 90 Propylene carbonate 50.5

[0070] TABLE 3 Applied Internal Change in voltage Capacitance resistancecapacitance Ex. 1 2.5 V 2.49 F  8.6Ω −12.4% Ex. 2 2.8 V 2.61 F  8.8Ω−17.9% Ex. 3 2.5 V 4.20 F 100 m −15.4% Ex. 4 3.0 V 2.78 F 15.7Ω −14.5%Ex. 5 3.0 V 2.81 F  8.8Ω −13.6% Ex. 6 3.0 V 2.79 F  9.2Ω −14.5% Ex. 73.0 V 2.81 F  8.8Ω −15.9% Ex. 8 3.0 V 2.85 F  8.3Ω −15.0% Ex. 9 2.5 V2.89 F  7.1Ω −12.9% Ex. 10 2.5 V 2.50 F  8.6Ω −16.7% Ex. 11 2.8 V 2.59 F 9.0Ω −21.8% Ex. 12 2.5 V 4.15 F 100 mΩ −20.1% Ex. 13 3.0 V 2.79 F 15.5Ω−19.7% Ex. 14 2.5 V 2.49 F  9.1Ω −14.2% Ex. 15 2.5 V 2.48 F  9.0Ω −13.7%Ex. 16 3.0 V 2.81 F  8.5Ω −19.2% Ex. 17 3.0 V 2.81 F  8.0Ω −22.1% Ex. 182.5 V 2.84 F  7.6Ω −15.4%

[0071] As is evident from Tables 1 and 2, the electric double layercapacitors of the present invention show higher voltage retention thanComparative Examples employing electrolytic solutions containing nofluorobenzene. Further, as is evident from Table 3, the electric doublelayer capacitors of the present invention show a less decrease incapacitance when a voltage is applied at 70° C. and thus have excellentreliability.

[0072] According to the present invention, it is possible to provide anelectric double layer capacitor having high voltage retention andcharacteristics excellent in reliability.

[0073] The entire disclosure of Japanese Patent Application No.2002-119059 filed on Apr. 22, 2002 including specification, claims andsummary is incorporated herein by reference in its entirety.

What is claimed is:
 1. An electric double layer capacitor having a pairof polarized electrodes and an electrolytic solution capable of formingan electric double layer at the interface with the polarized electrodes,wherein the electrolytic solution is an organic electrolytic solutioncontaining a fluorobenzene of the formula 1:

wherein n is an integer of from 1 to
 6. 2. The electric double layercapacitor according to claim 1, wherein the fluorobenzene is containedin an amount of from 0.1 to 30% in the total mass of the electrolyticsolution.
 3. The electric double layer capacitor according to claim 1,wherein the solvent in the electrolytic solution is at least one memberselected from the group consisting of propylene carbonate, butylenecarbonate, sulfolane, dimethyl carbonate and methylethyl carbonate. 4.The electric double layer capacitor according to claim 2, wherein thesolvent in the electrolytic solution is at least one member selectedfrom the group consisting of propylene carbonate, butylene carbonate,sulfolane, dimethyl carbonate and methylethyl carbonate.
 5. The electricdouble layer capacitor according to claim 1, wherein the cation of theelectrolyte in the electrolytic solution is a cation of the formula 2:R¹R²R³R⁴N⁺  Formula 2 wherein each of R¹, R², R³ and R⁴ which areindependent of one another, is a methyl group, an ethyl group or an-propyl group, provided that two selected from R¹ to R⁴ may togetherform a tetramethylene group.
 6. The electric double layer capacitoraccording to claim 1, wherein the cation of the electrolyte of theelectrolytic solution is a cation of the formula 3:

wherein each of R⁵ and R⁶ which are independent of each other, is a C₁₋₃alkyl group.
 7. The electric double layer capacitor according to claim1, wherein the cation of the electrolyte in the electrolytic solution isa cation of the formula 4: R⁷R⁸R⁹R¹⁰N⁺  Formula 4 wherein R⁷ is amethoxyalkyl group of the formula —(CH₂)_(n)OCH₃, wherein n is aninteger of from 1 to 3, and each of R⁸, R⁹ and R¹⁰ which are independentof one another, is a methyl group or an ethyl group, provided that twoselected from R⁸ to R¹⁰ may together form a tetramethylene group.
 8. Theelectric double layer capacitor according to claim 1, wherein the anionof the electrolyte in the electrolytic solution is an anion selectedfrom the group consisting of BF₄ ⁻, PF₆ ⁻, CF₃SO₃ ⁻and (CF₃SO₂)₂N⁻. 9.The electric double layer capacitor according to claim 8, wherein anelectrolyte of the formula 5 or 6:

is contained in an amount of from 30 to 60%, monofluorobenzene iscontained in an amount of from 0.1 to 30%, and dimethyl carbonate iscontained in an amount of from 20 to 69%, in the total mass of theelectrolytic solution.
 10. The electric double layer capacitor accordingto claim 9, wherein ethylmethyl carbonate is contained in an amount offrom 0.1 to 30% in the total mass of the electrolytic solution.
 11. Theelectric double layer capacitor according to claim 8, wherein anelectrolyte of the formula 7:

is contained in an amount of from 15 to 60%, monofluorobenzene iscontained in an amount of from 0.1 to 30%, and propylene carbonate iscontained in an amount of from 10 to 84%, in the total mass of theelectrolytic solution.
 12. An organic electrolytic solution comprising afluorobenzene of the formula 1:

wherein n is an integer of from 1 to 6, and an electrolyte having atleast one cation selected from the group consisting of the formula 2:R¹R²R³R⁴ N⁺  Formula 2 wherein each of R¹, R², R³ and R⁴ which areindependent of one another, is a methyl group, an ethyl group or an-propyl group, provided that two selected from R¹ to R⁴ may togetherform a tetramethylene group, the formula 3:

wherein each of R⁵ and R⁶ which are independent of each other, is a C₁₃alkyl group, and the formula 4: R⁷R⁸R⁹R¹⁰N⁺  Formula 4 wherein R⁷ is amethoxyalkyl group of the formula —(CH₂)_(n)OCH₃, wherein n is aninteger of from 1 to 3, and each of R⁸, R⁹ and R¹⁰ which are independentof one another, is a methyl group or an ethyl group, provided that twoselected from R⁸ to R¹⁰ may together form a tetramethylene group. 13.The organic electrolytic solution according to claim 12, wherein thefluorobenzene is contained in an amount of from 0.1 to 30% in the totalmass.
 14. The organic electrolytic solution according to claim 12, whichcontains at least one solvent selected from the group consisting ofpropylene carbonate, butylene carbonate, sulfolane, dimethyl carbonateand methylethyl carbonate.
 15. The organic electrolytic solutionaccording to claim 13, which contains at least one solvent selected fromthe group consisting of propylene carbonate, butylene carbonate,sulfolane, dimethyl carbonate and methylethyl carbonate.
 16. The organicelectrolytic solution according to claim 12, wherein the anion of theelectrolyte is an anion selected from the group consisting of BF₄ ⁻, PF₆⁻, CF₃SO₃ ⁻ and (CF₃SO₂)₂N⁻.
 17. The organic electrolytic solutionaccording to claim 16, wherein an electrolyte of the formula 5 or 6:

is contained in an amount of from 30 to 60%, monofluorobenzene iscontained in an amount of from 0.1 to 30%, and dimethyl carbonate iscontained in an amount of from 20 to 69%, in the total mass.
 18. Theorganic electrolytic solution according to claim 17, wherein ethylmethylcarbonate is contained in an amount of from 0.1 to 30% in the totalmass.
 19. The organic electrolytic solution according to claim 16,wherein an electrolyte of the formula 7:

is contained in an amount of from 15 to 60%, monofluorobenzene iscontained in an amount of from 0.1 to 30%, and propylene carbonate iscontained in an amount of from 10 to 84%, in the total mass.